Idle issue 86 760 Turbo

I am working an issue with an 86 760 Turbo red block 4 cyl. gasoline. Bought the car in November with no issues, this has presented in the last couple of weeks. The engine starts and runs fine but idles in the 1500-2000 rpm range, up significantly from the 800 rpm it was idling when I bought it. The MPG has gone from around 18 mpg to 13 mpg since the issue started. The idle drops back to around 800 if I blip the accelerator but jumps right back up to 1500-1800. The issue has self corrected twice since it started, meaning the engine operation has returned to normal two times without any adjustment. The issue reappeared after restart.
I'm thinking throttle position sensor because of the MPG decrease but would like to hear some troubleshooting techniques or things to check before I turn this over to the mechanic. Thanks.

Definitely check for vacuum leaks.

How's the engine wiring harness?

Harness looks like new. The car is a 93k survivor from an estate. Garage kept, very nice car. I'm sure it's something simple.

Check you idle motor start the car while its idling high try smacking the idle motor if idle drops.you have a sticky idle motor.

This issue has self corrected again. It seems to be a malfunction in the startup flare process. When working correctly, the engine at startup flares to about 1800rpm and drops back to 800rpm in a second or two. When malfunctioning, it stays at 1800 and fuel economy drops to 12-13 mpg. It is exactly like a sticking choke on an older carburated engine.

Is there a mechanical or electronic system that governs the engine speed and air flow at startup? Is it a part of the ECU routine, throttle position sensor, coolant temperature sensor, or MAF?

I know this will again malfunction or fail and would like to fix whatever is causing the problem. I appreciate any ideas or suggestions to help me solve this.

Thanks.

Check the coolant temp sensor, the one for the fuel system. If it tells the ECU the engine is cold the idle will be high and it will run rich also check it's wiring.

What are the weather conditions when this happens? Sun shining? Raining? Snowing?

I'll check the CTS, thanks for your response.

This happened initially Fall of last year, temps from 35-65 degrees... Happened again in winter, temps of -5 to 45. All precips, snow, rain, didn't seem to be affected by weather.

Thanks for your response.